=Thigh Muscles.=--Of the thigh muscles only a few need be mentioned. One large muscle is the _psoas magnus_, which has its origin on the front of the last dorsal and all the lumbar vertebræ, passes down across the brim of the pelvis and under Poupart’s ligament, gradually diminishing in size, and terminates in a tendon that is inserted into the lesser trochanter. It serves to flex the thigh on the pelvis and to rotate it outward. The _psoas parvus_ rises from the last dorsal and the first lumbar vertebræ and does not go out of the pelvis. The _sartorius_ or _tailor muscle_ is flat and ribbon-like and is the longest muscle in the body. It rises from the anterior superior spine of the ilium and is inserted into the upper inner surface of the shaft of the tibia. By it the legs are crossed. It also forms the outer side of an important landmark, _Scarpa’s triangle_, whose base is formed by Poupart’s ligament and the inner side by the _adductor magnus muscle_, which passes from the ramus of the os pubis and the tuberosity of the ischium to the linea aspera. The _femoral artery_ bisects the triangle and runs into its apex. The bulk of the anterior portion of the thigh is formed by the _quadriceps extensor_, which is really made up of four muscles, the _rectus femoris_, whose origin is on the anterior inferior iliac spine and above the acetabulum; the _vastus externus_, which comes from the greater trochanter and the upper linea aspera; and the _vastus internus_ and _crureus_, which rise from the neck of the femur and the linea aspera. It is inserted into the tubercle of the tibia by the _ligamentum patellæ_, in which the patella lies. Its action is to extend the leg. At the back and forming the _buttocks_ are the three glutei muscles, the _gluteus maximus_, _medius_, and _minimus_. All these rise from the outer side of the ilium and have their insertion on or about the great trochanter. They serve to hold the trunk erect and to extend, abduct, and rotate the thigh. [Illustration: FIG. 75.] [Illustration: FIG. 76.] [Illustration: FIG. 77. FIG. 75.--Superficial muscles of hip and thigh (from behind): 1, Gluteus medius; 2, gluteus maximus; 3, vastus externus; 4, biceps flexor cruris; 5, semitendinosus; 6, semimembranosus; 7, gracilis; 8, sartorius; 9, adductor magnus; 10, 11, gastrocnemius; 12, origin of plantaris. (Dorland’s Dictionary.) FIG. 76.--Muscles of the inner side of thigh and interior of pelvis: 1, Iliacus; 2, psoas magnus; 3, obturator internus; 4, pyriformis; 5, erector spinæ; 6, gluteus maximus; 7, sartorius; 8, adductor longus; 9, gracilis; 10, adductor magnus; 11, semimembranosus; 12, semitendinosus; 13, rectus femoris; 14, vastus internus. (Dorland’s Dictionary.)